Water Treatment

There are at least three ways to protect water quality in water supply systems:

• Use a relatively pristine natural source
• Manufacture a pristine source through storage reservoirs
• Filter or treat nonpristine water sources

Historically, water providers have tended to take a supply-side perspective, always seeking to solve quantitative or qualitative shortage issues by extending their supply systems deeper into remaining pristine sources (first strategy above), but over time they have been forced to enact the second and third strategies. As explored by other articles in this volume, strategy two dominated water management practices until the last quarter of the 20th century. Some commentators now believe that the time has long since passed when such supply-side strategies were appropriate and that demand-side management (forcing/encouraging water users to use water more efficiently) is required.

[Page 469]Water pollution sources may be split into “point” and “nonpoint” (or diffuse) sources. Comparison of the share of nonpoint and point sources in water pollution in the United States demonstrates the high degree of variability of pollution by source—some areas being dominated by one or the other, or perhaps both. Heavy metals such as cadmium and mercury can often enter the environment primarily through diffuse mechanisms, whereas organic pollution is usually localizable; that is, traceable to a source. Nonpoint source pollution is, by definition, harder to treat at source, and so treatment options tend toward reliance on the natural self-purification processes attendant on surface water and groundwater and on industrial wastewater collection, treatment, and release back into the natural environment. Urbanization also results in a certain amount of surface and groundwater contamination. For example, groundwater quality in the Marlborough and Berkshire Downs and the Kennet Valley in southern England is monitored using a network of public supply and private abstraction boreholes. In all cases there remains a heavy (over)reliance on the age-old water engineering mantra: “dilution is the solution to pollution.”

The management of phosphate pollution provides a good example of problems attendant on a pollutant with both point and nonpoint sources. At this time, the European Union and its member states are looking closely at strategies for managing phosphates in surface and subsurface waters. Although the two primary sources of phosphates are well known (i.e., detergents and fertilizers), removing or reducing them from water can be much more difficult than for other pollutants. Several member states have banned phosphates in detergents, but others argue that the cost of the measure is disproportional to the potential benefit, whereas in other states, the contribution from agriculture or even from sewage effluent is more significant and more difficult to manage. Some member states, including Germany, Italy, Hungary, and the Netherlands, have phased out phosphorus-based detergent activators and have seen gradual recovery of surface water quality.

It has already been mentioned that natural self-purification processes occur in groundwater and surface water, which can lead to improvements of its quality. Self-purification of surface water is the complete set of biological, physical, chemical, and hydrological processes leading to reduction of water pollution in water courses and reservoirs through biodegradation, sedimentatation, reaeration, dilution, and adsorption. The microorganisms that participate in the self-purification process consist primarily of bacteria: aerobes (which require the presence of oxygen dissolved in water) and anaerobes (which do not occur in the presence of oxygen, but take oxygen from chemical compounds). In aerobic conditions, decomposable organic compounds (carbohydrates, fats, and proteins) give the following final products: carbon dioxide, water, nitrates, and phosphates. In anaerobic conditions, the following are produced: methane, ammonia, hydrogen sulfite, alcohols, and organic acids. The aerobic decomposition of pollution is a much more intensive and beneficially productive process that does not produce as many toxic decomposition products (though too much of anything can be toxic in certain circumstances). Self-purification of water sources is usually described through the well-known Streeter-Phelps

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